1. Field of the Invention
This invention relates generally to elastic boundary wave devices, and more particularly, to an elastic boundary wave device that can suppress an unnecessary response.
2. Description of the Related Art
There are known surface acoustic wave (hereinafter, referred to as SAW) devices, as one of the devices that utilize elastic waves. The SAW devices are for use in various circuits such as, for example, transmit bandpass filters, receive bandpass filters, local filters, antenna duplexers, intermediate frequency (IF) filters, and frequency modulation (FM) modulators, in which wireless signals ranging from 45 MHz to 2 GHz in the frequency band are processed. Those wireless signals are typically used by, for example, mobile telephones.
In recent years, there have been developed elastic boundary wave devices that utilized elastic boundary waves. In an elastic boundary wave device, elastic waves are concentrated in the boundary of two different mediums. Therefore, even if a foreign material is adhered to outer surfaces of the two mediums, there will be no change or degradation in the characteristics, such as the variation of the frequency or the increase in the electric loss, unlike the SAW device.
In the SAW device, there is a problem of an unnecessary response that is never used by a resonator. Therefore, as disclosed in Japanese Patent Application Publication No. 6-112764 (hereinafter, referred to as Document 1), there is provided an absorber or grating at an edge of the substrate in the SAW device to suppress the unnecessary surface waves that have been reflected. Also, as disclosed in Japanese Patent Application Publication No. 4-239210 (hereinafter, referred to as Document 2), the reflection of bulk waves is suppressed by providing a sound absorbing material at an end surface of the substrate of the SAW device. In addition, as disclosed in Japanese Patent Application Publication No. 4-82315 (hereinafter, referred to as Document 3), there is a technique for suppressing the reflection of the bulk wave by providing a step at an end surface of the substrate of the SAW device.